MAX15021 Maxim Integrated Products, MAX15021 Datasheet - Page 15

• Manufacturer Part Number: MAX15021
• Description: Step-Down DC-DC Regulator
• Manufacturer: Maxim Integrated Products

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Below are equations that define the power modulator:
R
resonant break frequency of the filter, and f
ESR zero of the output capacitor. See the Closed-Loop
Response and Compensation of Voltage-Mode
Regulators section for more information on f
The switching frequency (f
between 500kHz and 4MHz. Typically, the crossover
frequency (f
closed-loop gain is equal to unity (crosses 0dB)—
should be set at or below one-tenth the switching fre-
quency (f
The MAX15021 provides an internal voltage-mode error
amplifier with its inverting input and its output available to
the user for external frequency compensation. The flexi-
bility of external compensation for each controller offers
a wide selection of output filtering components, especial-
ly the output capacitor. For cost-sensitive applications,
Figure 4a. Power Modulator Gain and Phase Response with
Lossy Bulk Output Capacitor(s) (Aluminum)
OUT
f
Regulator with Tracking/Sequencing Capability
LC
=
is the load resistance of the regulator, f
2
π
-60
-20
-40
-80
SW
Gain
×
40
20
0
10
CO
/10) for stable closed-loop response.
L C
|G
MOD(DC)
×
MOD
)—the frequency at which the system’s
< G
| ASYMPTOTE
f
100
OUT
ESR
______________________________________________________________________________________
MOD
×
=
1
1k
=
⎛
⎜
⎝
FREQUENCY (Hz)
2
R
R
π
V
f
LC
V
OUT
Dual, 4A/2A, 4MHz, Step-Down DC-DC
OUT
RAMP
×
AVIN
10k
ESR C
f
ESR
+
+
1
SW
DCR
ESR
100k
×
=
|G
V
V
) is programmable
OUT
MOD
AVIN
AVIN
⎞
⎟
⎠
4
MAX15021 fig04a
1M
|
≈
2
=
π
10M
4V/V
×
LC
90
45
0
-45
-90
-135
-180
L C
ESR
and f
1
×
LC
OUT
is the
is the
ESR
.
use aluminum electrolytic capacitors while for space-
sensitive applications, use low-ESR tantalum or multilay-
er ceramic chip (MLCC) capacitors at the output. The
higher switching frequencies of the MAX15021 allow the
use of MLCC as the primary filter capacitor(s).
First, select the passive and active power components
that meet the application output ripple, component
size, and component cost requirements. Second,
choose the small-signal compensation components to
achieve the desired closed-loop frequency response
and phase margin as outlined below.
The power modulator’s LC lowpass filter exhibits a vari-
ety of responses, dependent on the value of the L and
C and their parasitics. Higher resistive parasitics
reduce the Q of the circuit, reducing the peak gain and
phase of the system; however, efficiency is also
reduced under these circumstances.
One such response is shown in Figure 4a. In this exam-
ple, the ESR zero occurs relatively close to the filter’s
resonant break frequency, f
modulator’s uncompensated crossover is approximate-
ly one-third the desired crossover frequency, f
also, the uncompensated rolloff through the 0dB plane
follows a single-pole, -20dB/decade slope, and 90° of
phase lag. In this instance, the inherent phase margin
ensures a stable system; however, the gain-bandwidth
product is not optimized.
Figure 4b. Power Modulator and Type II Compensator Gain and
Phase Response with Lossy Bulk Output Capacitor(s) (Aluminum)
Closed-Loop Response and Compensation
-20
-40
-60
-80
80
60
40
20
0
10
|G
EA
|
100
< G
<G
EA
MOD
1k
FREQUENCY (Hz)
of Voltage-Mode Regulators
10k
f
f
LC
ESR
LC
. As a result, the power
100k
|G
MOD
f
CO
|
MAX15021 fig04b
1M
10M
180
135
90
45
0
-45
-90
-135
-180
CO
. Note
15